1. Field of the Invention
The invention relates to a reflection-type liquid crystal display having a reflecting plate for reflecting out a light transmitted through a liquid crystal layer from an outside and a method for manufacturing a same.
2. Description of the Related Art
Reflection-type liquid crystal displays have been used mainly in a portable terminal because they can be made thinner, less power consuming, and lighter in weight than transmission-type ones. Specifically, a reflecting plate in the reflection-type liquid crystal display reflects an incident light transmitted from the outside, and it is therefore available as a light source for display, thus eliminating a necessity of a back-light.
A recent reflection-type liquid crystal display includes basically a liquid crystal of, for example, a TN (Twisted Nematic)-type, a single polarizing plate-type, a STN (Super Twisted Nematic)-type, a GH (guest host)-type, or a PDLC (Polymer Dispersion)-type, a Cholesteric-type, or alike, a switching element for driving the liquid crystal, and a reflecting plate provided inside or outside a liquid crystal cell. Such a typical reflection-type liquid crystal display employs an active matrix scheme which realizes high definition and high picture quality by using a TFT (TFT) or metal/insulator film/metal-structured diode (MIM) as the switching element and also has the reflecting plate attached thereto.
The following will describe a conventional liquid crystal display of the single polarizing plate-type with reference to FIG. 19.
An opposed-side substrate 1 includes a polarizing plate 2, a phase-difference plate 3, a glass substrate 4, a color filter 5, and a transparent electrode 6. A lower side substrate 7 includes, on the other hand, a glass substrate 8, a reverse stagger-structured TFT 9 formed as a switching element on the glass substrate 8, a protrusion shape 10 made up of a first insulating layer which provides an unevenly-structured base, a polyimide film 11 formed thereon as a second insulating layer, and a reflecting electrode 13 which is connected to a source electrode 12 of the TFT 9 and also which functions as a reflecting plate-and-pixel electrode.
Between the opposed-side substrate 1 and the lower side substrate 7 is located a liquid crystal layer 14.
A reflected light 16 is utilized for display. The reflected light 16 is given by an incident light 15 from outside when it passes through the polarizing plate 2, the phase-difference plate 3, the glass substrate 4, the color filter 5, the transparent electrode 6, and the liquid crystal layer 14 and then is reflected by the reflecting electrode 13.
This reflection-type liquid crystal display needs to have such display performance that it would give bright and white display when the liquid crystal is in a light-transmitting state. To achieve this display performance, the incident light 15 transmitted in various orientations needs to be efficiently emitted to the outside. To do so, an uneven structure can be formed on the polyimide film 11 to thereby provide the reflecting electrode 13 located thereon with a light-scattering function. Therefore, the display performance of the reflection-type liquid crystal display largely depends on how to control the uneven structure of the reflecting electrode 13.
The following will describe a conventional method for manufacturing a reflecting electrode used in the conventional reflection-type liquid crystal display with reference to FIG. 20A and FIG. 21J.
In steps for manufacturing a TFT, first a gate electrode 21 is formed on a glass substrate 20 (FIG. 20A). Subsequently, a gate insulator film 22, a semiconductor layer 23, and a doping layer 24 are formed (FIG. 20B). Subsequently, an island 25 of the semiconductor layer 23 and the doping layer 24 is formed (FIG. 20C), thereby forming a source electrode 26 and a drain electrode 27 (FIG. 20D). Next, a reflecting electrode 34 is formed.
In steps for manufacturing the reflecting electrode, first an organic insulator film 28 is formed which has photosensitivity (FIG. 20E). Subsequently, protrusions 29 are formed by photolithography in a region for forming the reflecting electrode (FIG. 20F) and melted into a smooth protrusion shape 30 (FIG. 21 G). Subsequently, the smooth protrusion shape 30 is covered with an organic insulator film 31 to form a further smoother uneven surface 32 (FIG. 21H). Subsequently, to electrically connect the reflecting electrode to the source electrode of the TFT, a contact portion 33 is formed (FIG. 21I), to then form a reflecting electrode 34 (FIG. 21J). This method for manufacturing reflecting electrodes is disclosed for example in Japanese Examined Patent Application No. Sho 61-6390 and in Proceedings of the SID (Tohru Koizumi and Tatsuo Uchida, Vol. 29, p. 157, 1988).
FIG. 22 is a plan view of a pattern of the protrusions 29 in the FIG. 20F. The following will describe the process with reference to FIG. 22F.
Protrusions 29 are not in contact with each other, that is mutually isolated. The protrusions 29 are each extremely small, measuring 1-20 xcexcm in diameter and 0.5-5.0 xcexcm in height. Therefore, during a subsequent substrate washing process, a heating process, or a film forming process, adherence between the protrusions 29 and underlying layer may deteriorate, thus causing the protrusions 29 to problematically flake off.
With this, therefore, a desired uneven structure cannot be formed in a reflecting electrode region, so that a desired optical property cannot be obtained for the reflecting electrode. That is, such the reflection electrode, if used in the reflection-type liquid crystal display, would give dark display or irregularities in brightness.
To prevent flake-off of the protrusions, also, it may be suggested that a coupling material be applied under the protrusions 29 to improve adherence. Under and below the protrusions 29, however, the TFT, the wiring lines, and a like are arranged, so that they may be adversely influenced by the coupling material, thus deteriorating reliability of the switching element. Therefore, the coupling material should not be used.
In view of the above, it is an object of the invention to provide a reflection-type liquid crystal display which prevents flake-off of protrusions which provide a base for the uneven structure of a reflecting electrode to thereby achieve high brightness and high definition display performance, and a method for manufacturing same.
According to a first aspect of the present invention, there is provided a reflection-type liquid crystal display including:
a transparent first substrate;
a transparent electrode provided on the transparent first substrate;
a second substrate;
an insulator film which is provided on the second substrate and also on a surface of which is formed an uneven structure;
a reflecting electrode which is provided on the insulator film in such a shape as reflecting the uneven structure; and
a liquid crystal layer sandwiched by a side of the transparent electrode formed on the first substrate and a side of the reflecting electrode provided on the second substrate;
wherein the insulator film includes a first insulating layer in which a large number of depressions are irregularly arranged which are isolated as surrounded by protrusions and a second insulating layer which covers the first insulating layer entirely.
In the foregoing first aspect, the depressions refer to portions where there is essentially no film thickness present and so may be called apertures, through-holes or a like instead.
Protrusions on the first insulating layer according to a prior art are not in contact with each other, that is, are isolated. Therefore, if some of all the protrusions have weaker adherence with the underlying layer, they easily flake off. The protrusions on the first insulating layer according to the first aspect are all connected in a network. Therefore, even if some of those protrusions have a weaker adherence with an underlying layer, they may be supported by surrounding protrusions. With this, the protrusions can be prevented from flaking off.
In other words, the protrusions on the first insulating layer according to the first aspect are formed by an irregular arrangement of isolated depression patterns. Since the protrusions on the first insulating layer according to the prior art are formed by an irregular arrangement of isolated columnar protrusions, they easily flake off during subsequent manufacturing processes. With the first aspect the isolated depression patterns are irregularly arranged to thereby increase a contact area between the protrusions and the underlying layer, so that the protrusions do not easily flake off during subsequent manufacturing processes.
Also, those protrusions may be formed by an irregular arrangement of stripe-shaped protrusion patterns. If, in this case, the protrusions are formed by an irregular arrangement of stripe-shaped protrusion patterns, they have a larger contact area with the underlying layer than the columnar protrusion patterns according to the prior art, thus improving adherence.
Also, in the above-mentioned uneven structure, irregular uneven shapes may be repeatedly formed in an entire region of a reflecting electrode in units of one pixel (picture element) or more. With this, it is possible to suppress interference of reflecting properties, so that the reflection-type liquid crystal display employing this reflecting electrode is free of wavelength dependency without deterioration in color properties, thus providing bright and high-definition display performance.
Also, the above-mentioned protrusions may be melted into a smooth sectional shape. Next, these protrusions are covered by the second insulating layer formed subsequently, to obtain the uneven structure, so that the reflecting electrode formed thereon has good optical reflecting properties, thus permitting the reflection-type liquid crystal display having this reflecting electrode in the liquid crystal cell to give brighter display.
Also, the above-mentioned first or second insulating layer can act also as a protection film for a switching element, to prevent it from being contaminated from outside, thus achieving stable switching operations.
Also, at least one of the first and second insulating layers can cover wiring lines (at least one of drain and gate wiring lines), to reduce a parasitic capacitance due to the wiring lines and the reflecting electrode, thus suppressing occurrence of cross-talk or a like in the reflection-type liquid crystal display.
Also, at least one of the first and second insulating layers has photo-absorbancy to thus absorb an incident light from between mutual reflecting electrodes. With this, the incident light can be prevented from being applied to the switching element to thereby good switching properties, thus resulting in the reflection-type liquid crystal display having high contrast and high brightness display properties.
Also, at least one of the first and second insulating layers may have a contact hole made therein for electrically interconnecting the reflecting electrode and underlying switching element. In this case, the reflecting electrode can be provided at a top of a pixel and so can be increased in area to achieve a higher numerical aperture, thus implementing the reflection-type liquid crystal display having brighter display performance.
Also, by forming the protrusions of an organic or inorganic material having photosensitivity, patterning step for forming the protrusions can be shortened. Also, by forming the second insulating layer of an organic or inorganic material having photosensitivity, the patterning step for forming contact pattern can be shortened to thereby simplify process required, thus reducing cost for manufacturing the reflection-type liquid crystal display.
According to a second aspect of the present invention, there is provided a reflection-type liquid crystal display manufacturing method for forming an uneven structure in the reflection-type liquid crystal display according to the first aspect, the manufacturing method including steps of:
forming a first insulating layer of an organic or inorganic insulating material having photosensitivity;
forming an uneven-element pattern on the first insulating layer by photo-exposure;
etch-developing on the first insulating layer;
melting by heat treatment the first insulating layer thus etch-developed, to thereby smooth an uneven structure; and
forming a second insulating layer on the first insulating layer thus melted.
With the above second aspect, it is possible to omit the resist applying, flaking, and etching steps of the resist process in patterning of the depression-protrusion portion (step), thus simplifying process and reducing costs of the reflection-type liquid crystal display. In addition, a smooth and continuous uneven structure can be manufactured, thus implementing a reflecting electrode which has a uniform and uneven surface free of protrusion flake-off.